mperfectly known, that they give little
help in determining the descent of the class, but, on the other
hand, certain reptilian orders of the Permian period, especially
well represented in South Africa, display so many and such close
approximations to mammalian structure, as strongly to suggest a genetic
relationship. It is difficult to believe that all those likenesses
should have been independently acquired and are without phylogenetic
significance.

Birds are comparatively rare as fossils and we should therefore look in
vain among them for any such long and closely knit series as the
mammals display in abundance. Nevertheless, a few extremely fortunate
discoveries have made it practically certain that birds are descended
from reptiles, of which they represent a highly specialised branch. The
most ancient representative of this class is the extraordinary genus
Archaeopteryx from the upper Jurassic of Bavaria, which, though
an unmistakable bird, retains so many reptilian structures and
characteristics as to make its derivation plain. Not to linger over
anatomical minutiae, it may suffice to mention the absence of a horny
beak, which is replaced by numerous true teeth, and the long lizard-like
tail, which is made up of numerous distinct vertebrae, each with a pair
of quill-like feathers attached to it. Birds with teeth are also found
in the Cretaceous, though in most other respects the birds of that
period had attained a substantially modern structure. Concerning
the interrelations of the various orders and families of birds,
palaeontology has as yet little to tell us.

The life of the Mesozoic era was characterised by an astonishing number
and variety of reptiles, which were adapted to every mode of life,
and dominated the air, the sea and the land, and many of which were
of colossal proportions. Owing to the conditions of preservation which
obtained during the Mesozoic period, the history of the reptiles is a
broken and interrupted one, so that we can make out many short series,
rather than any one of considerable length. While the relations of
several reptilian orders can be satisfactorily determined, others still
baffle us entirely, making their first known appearance in a fully
differentiated state. We can trace the descent of the sea-dragons, the
Ichthyosaurs and Plesiosaurs, from terrestrial ancestors, but the most
ancient turtles yet discovered show us no closer approximation to any
other order than do the rec